Non-volatile memory systems, such as flash memory, have been widely adopted for use in consumer products. Flash memory may be found in different forms, for example in the form of a portable memory card that can be carried between host devices or as a solid state disk (SSD) embedded in a host device. When writing data to a conventional flash memory system, a host typically writes data to, and reads data from, addresses within a logical address space of the memory system. The memory system then commonly maps data between the logical address space and the physical blocks or metablocks of the memory, where data is stored in fixed logical groups corresponding to ranges in the logical address space. Generally, each fixed logical group is stored in a separate physical block of the memory system. The memory system keeps track of how the logical address space is mapped into the physical memory but the host is unaware of this. The host keeps track of the addresses of its data files within the logical address space but the memory system generally operates without knowledge of this mapping.
A drawback of memory systems that operate in a logical address space, also referred to as logical block address (LBA) format, is fragmentation. Data written by a host file system may often be fragmented in logical address space, where many fixed logical groups are only partially updated with new data. The fragmentation may occur as a result of cumulative fragmentation of free space by the host file system, and possibly even as a result of inherent fragmentation of individual files by the host file system. Data previously written may become obsolete due to deletion and cause further fragmentation. The fragmented logical groups will need to be rewritten in full in a different physical block. The process of rewriting the fragmented logical groups may involve copying unrelated data from the prior location of the logical group.
Fragmentation may also increase if data is written randomly followed by data written sequentially in the logical address space. The increased fragmentation may result in a higher than desired write amplification ratio, i.e., the ratio of the amount of data programmed, including internal copying/movement of data, relative to the amount of data written by a host. The overhead due to the increased fragmentation and write amplification ratio can result in lower performance and reduced device lifetime for the memory system.